The Skeletal System: Bones, Joints, and Connective Tissue
Movement of the body is possible because of the interaction between the muscular system and the skeletal system. Together these two systems are referred to as the musculoskeletal system, which consists of bones, joints, connective tissue (ligaments and tendons), and the muscles. Muscles alone do not move weights, but rather they function by moving bones that rotate around connective tissue. Our bones provide the structural support and our muscles convert chemical energy into mechanical energy for movement. Our joints transmit forces through the bones of our body to the external environment. In this article I will discuss the skeletal system. To read about the muscular system see my article “The Muscular Systems”.
The Skeletal System
The average human adult skeleton has 206 bones joined by ligaments and tendons. This forms a supportive and protective structure for underlying soft tissue and muscles. Our skeletal system has several important functions in our body:
1) Our bones server as levers that transmit muscular forces.
2) Our internal organs are protected by our skeletal system.
3) Our skeletal system acts as a framework for other tissues and organs.
4) Our bones store and release minerals like calcium and phosphorous.
Our skeletal system is broken down into two specific regions: the axial skeleton and the appendicular skeleton. The axial skeleton is comprised of 180 bones which include the skull, spine, ribs, and sternum. The appendicular skeleton comprises of 126 bones that are divided as follows: 60 in the upper extremities, 60 in the lower extremities, 3 in the pelvic girdle, and 4 in the shoulder girdle.
Bones of the Skeletal System
All of the 206 bones in our body consist of three layers: the bone marrow, compact bone, and the periosteum. Within the center section of our long bone is a central cavity which holds the bone marrow. The are two different types of bone marrow: 1) red marrow produces platelets to assist in blood clotting, and red blood cells which fight against infection, and 2) yellow marrow which consists mostly of fat cells. Surrounding the marrow is a dense rigid bone called the compact bone. The ridge compact bone is honeycombed with thousands of tiny holes and passages that supply oxygen and nutrients to the bone. This dense layer of compact bone supports the weight of the body and is mostly comprised of calcium and minerals. Every bone is then covered by the periosteum which acts as the skin of the bone. The inner layer of the periosteum contains cells the produce bone.
Our 206 bones of the skeletal system are divided into two categories: 1) the axial skeleton, which is comprised of our trunk and head, and 2) the appendicular skeleton, which is comprised of our arms and legs. Our bones are further broken down into five main categories:
1) Flat Bones: Provide protection. They include such bones as the ribs, sternum, and scapula.
2) Short Bones: Provide shock absorption. They include such bones as the carpals, and tarsals.
3) Long Bones: They provide the structural support of our body. They include such bones as the tibia, fibula, and ulna.
4) Sesamoid Bones: Provide protection and improve mechanical advantage of musculotendinous units (relating to or affecting both muscle and tendons). They include such bones as the patella (knee cap).
5) Irregular Bones: Provide a variety of purposes throughout the body. They include such bones as a vertebra.
Joints the Skeletal System
A joint, or otherwise known as an articulation; is formed where two bones connect. There are two major classifications of skeletal system joints: 1) synarthrodial joints, which is a union of two bones by fibrous tissues such that there is no joint cavity and almost no movement possible. An example would be the skull, and 2) diarthrodial joints, which is a freely moving joint with an articular cavity (which holds the fluid inside the joint).
As a freely moving joint, the diarthrodial joint has an articular cavity encased in a ligamentous capsule, and synovial fluid lubricates the cartilage inside the capsule. There are six categories of diarthrodial joints:
1) Arthrodial Joint (gliding): They comprise of two flat bone surfaces that press up against each other and allow a limited gliding movement. They can be found in the wrist and the foot.
2) Ginglymus Joint (hinge): They provide a wide range of movement in one place. An example is the knee joint.
3) Condyloid Joint (ellipsoid): They provide movement in two planes without rotation. An example is the wrist between the radius and the proximal row of carpal bones.
4) Enarthrodial Joint (multi-axial ball-and-socket): They permit movement in all planes. An example is the hip joint.
5) Sellar Joint (saddle): This joint provides movement similar to ball-and-socket movement but without rotation. The thumb is the only saddle joint in the body.
6) Trochoidal Joint (pivot): This is a joint that moves by rotating. One bone articulates just like the hinge on a door in such away that one bone rotates using the other as pivot. An example is the neck.
Connective Tissue of the Skeletal System
Connective tissues primary function in the skeletal system is to connect muscles to bone and to connect joints together. This dense connective tissues is comprised of fibers called collagen. Mature connective tissue has fewer cells than other tissues and needs less blood, oxygen, and other nutrients. Each collagen bundle is comprised of several fibers, and these fibers contain fibrils. The fibrils contain the actual collagen molecules.
Tendons connect muscles to bone and are an extension of the muscle fibers. They are slightly more elastic than ligaments but cannot shorten as muscles do. Within the muscles and tendons there is a built in sensory mechanisms called the Golgi tendon organ. The Golgi tendon organ acts as a “safety valve” and provides feedback about the bodies position and protects the muscle and connective tissue. This safety valve is called the feedback loop. In essence, when tension becomes to great, greater than the brain can recall; the Golgi tendon organ’s signal inhibits the contraction stimulus and reduces the risk of injury.
Ligaments of the skeletal system connect bones to bones at a joint. Ligaments contain collagen and also contain an elastic fiber called elastin. Ligaments do have some elasticity to allow joint movement, but it is very limited.
Cartilage of the skeletal system is the firm, elastic, flexible, white material found at the ends of the ribs, between the vertebrae, at joint surfaces, and in the nose and ears. Cartilage functions as both a shock absorber, and to provide structure. Cartilage also functions as a lubricant in the working parts of a joint. Unlike tendons and ligaments, cartilage has no blood supply of its own. It receives oxygen and nutrients through diffusion. Because of this, damage to cartilage takes a very long time to heal.
What does it all Mean for the Athlete?
There are positive effects on connective tissue with training and exercising. Exercise is known to increase bone density and improve bone health, and helps to maintain bone density at any age. It is also shown that weight-bearing exercise appears to stimulate bone formation and retain calcium in the bones that are bearing the load. The force of muscles pulling against bones stimulates this bone-building process. So any exercise that places force on a bone will strengthen that bone.
Physical training will increase the tensile strength, increase their size, and increase their resistance to injury. With proper training, you can alter the Golgi tendon organ and literally push back the safety valve. Training needs to be specific to alter the structure of connective tissue. Although endurance training has shown some adaptations, it takes high-speed ballistic movements to make any dramatic changes (as is power lifting or plyometrics).